With the rapid development of information communication technology in recent years, we are moving toward a ubiquitous society based on such information communication technology.
In order to provide access to information communication devices anytime and anywhere, it is necessary to install sensors each having a computer chip capable of performing a communication function in all social facilities. Consequently, the supply of power to such devices and sensors is a new issue. In addition, as the variety of kinds of portable devices, such as a mobile phone, a Bluetooth headset and an iPod music player, have increased, it takes time and effort for users to charge batteries. In recent years, wireless power transfer has attracted attention as a method of solving this problem.
Wireless power transmission or wireless energy transfer refers to a technology of wirelessly transferring electric energy from a transmitter to a receiver using the principle of magnetic induction. In the 1800's, an electric motor or a transformer employing the principle of electromagnetic induction was used, and since then attempts to realize methods for transmitting electric energy by emitting electromagnetic waves, such as radio waves, lasers, high-frequency waves, and microwaves, have been made. In practice, electrical toothbrushes or electrical razors, which are frequently used in daily life, are charged based on the principle of electromagnetic induction.
Wireless energy transfer schemes used to date may be classified into a magnetic induction scheme, an electromagnetic resonance scheme, and a power transfer scheme using a short-wavelength radio frequency.
The magnetic induction scheme is a technology using a phenomenon in which when two coils are disposed adjacent to each other and current is supplied to one of the coil, a magnetic flux is generated, and as a result electromotive force is generated in the other coil. The magnetic induction scheme is being rapidly commercialized for small-sized devices such as mobile phones. The magnetic induction scheme has an advantage in that it is possible to transfer power of a maximum of several hundreds of kilowatts (kW) with high efficiency. However, the magnetic induction scheme has a disadvantage in that the maximum transfer distance is 1 cm or less and thus coils must be disposed adjacent to a charger or the bottom of a mobile phone.
The electromagnetic resonance scheme is characterized by using an electric field or a magnetic field instead of utilizing an electromagnetic wave or current. The electromagnetic resonance scheme has an advantage in that the electromagnetic resonance scheme is not affected by the electromagnetic wave and is thus safe for other electronic devices and human bodies. However, the electromagnetic resonance scheme has disadvantages in that the electromagnetic resonance scheme is useful only within a limited distance and space and in that energy transfer efficiency is somewhat low.
The short-wavelength wireless power transfer scheme, simply referred to as an RF scheme, utilizes the principle whereby energy can be directly transmitted and received in a radio-wave form. This technology is an RF wireless power transfer scheme using a rectenna. “Rectenna”, which is a portmanteau word of “antenna” and “rectifier”, means an element for directly converting RF power into DC power. That is, the RF scheme is technology for converting an AC radio wave into DC. In recent years, research has been actively conducted on commercialization of the RF scheme with improvement in efficiency of the RF scheme.
Wireless power transfer may be variously utilized in various industries, such as the vehicle, IT, railway, and electric home appliance industries, in addition to the mobile communication industry.
In general, a wireless power transmitter is provided with a coil for wireless power transfer (hereinafter, referred to as a transmission coil), and various shielding members are used to prevent an electromagnetic field or AC power generated by the transmission coil from being transferred to a control board.
A magnetic shielding sheet or a sandust block, which is formed of magnetic metal powder, is used as a representative shielding member.
In addition, for a wireless power receiver, a shielding member is used to block an electromagnetic field received by a reception coil.
In the case in which a single magnetic shielding member is used, however, the efficiency of shielding of an imaginary part is low, whereby energy is lost.